It is standard to mine coal from a face by means of a piece of equipment comprising a longitudinal succession of roof props, a limitedly displaceable and deformable conveyor extending longitudinally along the face, and a seam cutter displaceable longitudinally along the face also. The cutter removes material from the face and deposits it in the conveyor which displaces it longitudinally back along the cut. The props each have a base part standing on the floor, a roof part engaging the ceiling, an actuator capable of pushing these parts vertically apart, and a retractable flap that can be extended against the roof toward the face by its own individual actuator. Basically the function of the support flaps is to hold up the ceiling ahead of and behind the seam cutter.
As the seam cutter is moved along the face the support flaps are fully retracted to give it space to work. Behind the seam cutter the props and the conveyor are moved closer to the face, which is moved transversely outward with each pass of the cutter.
Thus in the coal-mining system the operator of the seam cutter controls the system from his or her location behind the seam cutter protected by one of the props. For a cutting operation the operator pushes a start button that fully retracts all the support flaps of a group of props downstream, that is ahead in the cutting direction, of the seam cutter, so that these flaps are not in the way of the cutter. If necessary auxiliary flaps that press against the face are also retracted. Then the cutter is advanced downstream along the face, cutting away the face and depositing the coal, ore, and the like into the conveyor.
Afterward the operator extends the flaps upstream of the cutter to produce a safety or protected zone, these flaps being pushed right up against the face. Thereupon the conveyor is realigned adjacent the face by pushing it with the actuators of the props, which for this function are spanned tight against the roof and floor. Subsequently the props are pulled transversely over toward the conveyor by these actuators, during which time their vertical actuators are relaxed and their flaps are allowed to pull in.
A disadvantage of this system is that the control is always exercised groupwise, that is the succession of props are operated in groups that must each include the same number of such props. The minimum group size is determined by the minimum number of props that must be used to deflect the conveyor, and this number is in turn a function of the deflectability of this conveyor. Unfortunately, when the rock or earth in which the face is being cut is not particularly stable, this can lead to modest cave-ins upstream and downstream of the mining machine when the support flaps are retracted, as the amount of unsupported ceiling is just too extensive to hold.
Thus it is often necessary to reduce the group size to an absolute minimum. This means that the conveyor will be deflected very greatly, putting a substantial strain on the joints between the individual jointed segments of this conveyor. In addition the mining efficiency drops as a relatively small bit is taken with each forward step of the mining machine, or with each backward step depending on which way the machine is working.